As of linux-3.1.1-1 the device is detected by the qcserial module, if not, you're going to have to recompile the qcserial module with your added product and vendor id.

Alternatively you can add the Product and Vendor ID by writing them into the new_id file (best both at the same time because most Gobi modules switch the Product ID when the Firmware is loaded).
Here the Shell Commands i had to use (in a root shell, sudo doesnt work in this case for some reason):
my integrated Gobi2K has the Vendor ID 04da and the Product IDs 250e (waiting for Firmware) and 250f (firmware loaded)

gobi_loader

"gobi_loader is a firmware loader for Qualcomm Gobi USB chipsets. These devices appear in an uninitialised state when power is applied and require firmware to be loaded before they can be used as modems. gobi_loader adds a udev rule that will trigger loading of the firmware and make the modem usable." (http://www.codon.org.uk/~mjg59/gobi_loader/)

Connection

wvdial

The general procedure is to switch the device into modem mode, make sure the ttyUSB device(s) are recognized by the qcserial kernel module, and then to run wvdial to dial, connect and start pppd.

Install wvdial

# pacman -S wvdial

The configuration file /etc/wvdial.conf will in general depend on (a) which device you have (b) which mobile network you are connecting to. A single wvdial.conf file can be defined with named sections to be usable with several USB modems and networks, should you need them.

Run (as root)

# wvdialconf

which will attempt to write /etc/wvdial.conf correctly. You will need to add the user, password and Access Point Name (APN). You can obtain these (i) from your network provider,
(ii) from other users via published wvdial.confs, or (iii) by logging the USB tty traffic under another operating system (Sysinternals' Portmon).

To simplify the procedure, I took my SIM card out and disabled the PIN so I don't have to run "wvdial pin" before connecting to the internet.

Often there will be several devices (at /dev/ttyUSB0, /dev/ttyUSB1, /dev/ttyUSB2 for example). If in doubt about which to use, try each of them in turn. Once the configuration files are prepared, the internet connection is established by running

$ wvdial <section>

The final wvdial command should start pppd and the obained IP address should be visible in the terminal output. At that point the internet connection should be live, which can be easily checked with a web browser or by pinging an external IP address.